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An easy way to measure the radius is to measure the distance across the entire circle (called the diameter) and then cut that in half. Multiply that by itself, and then multiply the product by the number ?. The digits of ? go on forever, but here are the first 10 digits: 3.141592653. The more digits you use, the more accurate your answer becomes.

It is important not to get this equation mixed up with C=2?r, which gives you the circumference or the length of the edge of the circle. The equation for the area of a circle is also often confused with the equation A=4?r^2, which gives you the surface area of a sphere, or a perfectly round ball. A sphere is the three-dimensional counterpart of the circle; while a circle is made up of points in two-dimensional space that are equally far apart from the center, a sphere is made up of points in three-dimensional space that are all the same distance from the center.